In recent years, vehicles such as an HEV (hybrid electric vehicle) and an EV (electric vehicle) have been spreading. The HEV and the EV are mounted with a secondary battery. In the HEV, the vehicle is driven or the engine is assisted by driving a motor by using the power stored in the secondary battery. For this reason, in the HEV, switching between charging and discharging of the secondary battery is frequently repeated with the traveling. Since an overcharge or an overdischarge deteriorates the secondary battery, it is necessary to control charging and discharging while grasping the amount of charge of the secondary battery.
Moreover, to determine the deterioration of the secondary battery, it is necessary to accurately grasp the internal resistance of the secondary battery.
For example, an internal resistance estimation device has been disclosed that estimates the internal resistance of the secondary battery based on the time elapsed from the detection of switching from charging to discharging or discharging to charging of the secondary battery when it is determined that the magnitude of the charging/discharging current of the secondary battery continues for a predetermined time period or longer under a condition where it is within a predetermined range (see Japanese Patent No. 4923929).